1. Field of the Invention
The present invention relates to a receiver, and more particularly, to a balun apparatus and a receiver comprising the same.
2. Description of the Background Art
A circuit for processing a differential signal is essential in a highly integrated receiver to reduce the effects of external noise and switching noise generated in an inside digital circuit.
Therefore, it requires an element that transforms an input signal received through an antenna to a differential signal. Such an element is a balun.
As a typical balun, a passive balun using a transformer and an active balun using a transistor amplifier were introduced.
FIG. 1 shows a passive balun circuit in accordance with the related art.
As shown in FIG. 1, the passive balun circuit according to the related art is embodied by comprising a transformer. The passive balun circuit transforms an input signal (Single-ended; VIN) applied at the primary coil of the transformer 100 having Lp inductance to an in-phase output signal (VOUT+) and an inverse-phase output signal (VOUT−) through the secondary coil of the transformer 100 having Ls inductance.
The passive balun circuit has an advantage that small amplitude error and small phase error can be achieved for the in-phase output signal (VOUT+) and the inverse-phase output signal (VOUT−).
However, the power of the input signal is weakened because of an imperfect coupling coefficient (k-factor) that is a typical characteristic of the practical transformer 100 in the passive balun circuit. Thus, the power of the in-phase output signal (VOUT+) and an inverse-phase output signal (VOUT−) become weakened compared to that of the input signal. Usually in practice, the k-factor ranges from 0.5 to 0.8 in a real environment whereas an ideal k-factor is 1.
Therefore, the passive balun circuit cannot fully transfer the power of the input signal to the output terminals. That is, the passive balun circuit suffers the power loss due to the imperfect coupling coefficient. Consequently, the signal attenuation occurs through the balun, and it will directly degrade the noise figure and the power gain of the total receiving terminal since it is the most front-end element in the total receiving terminal.
FIG. 2 shows an active balun circuit in accordance with the related art.
Referring to FIG. 2, the active balun circuit 200 according to the related art comprises an input unit 210, a feed-back unit 220, and an output unit 230.
The input unit 210 is embodied as a differential circuit that comprises a first transistor NM2a and a second transistor NM2b. And the gate terminal of the second transistor NM2b is connected to the feed-back unit 220. The input signal VIN is applied to the gate terminal of the first transistor NM2a and the output signals VOUT+ and VOUT− are extracted differentially.
In FIG. 2, the input unit 210 is embodied as a common source circuit. However, the input unit 210 can be also embodied as a common gate circuit.
The active balun circuit 200 has drawbacks as follows. The active balun circuit 200 consumes greater power due to the transistors involved, and usually operates only in a narrow band due to the resonance circuit 220.